Boost converters have been widely used in various power conversion applications such as single phase and three phase power factor corrected AC/DC switch-mode rectifiers. The boost stage processes the AC input and develops a DC output voltage that is typically between 400 volts and 800 volts. Boost converters are usually the topology of choice for providing a high output voltage from substantially lower DC voltages derived from sinusoidal input voltages. Boost converters are often used to supply power to computer or telecommunication systems that require power which is both clean and well regulated. If the power is interrupted, these systems usually require a "shut down" period in which to orderly prepare themselves. This period varies with application and should assure that an orderly shut down is completed. Boost converters employ various schemes that allow them to maintain their output voltage for a period of time after the input power has failed. One of the more prevalent "holdup" schemes is to use a large output capacitor that stores sufficient energy to maintain the output voltage above a minimum level for a specified period of time. For certain applications, this scheme requires prohibitively large capacitors in terms of size.
Another scheme to maintain boost converter output voltage during holdup operation is to use a collection of switches to connect a portion of the output capacitance across the boost converter input during holdup mode operation. This allows the boost circuitry to transfer energy stored in this capacitance to the boost converter output thereby maintaining the output voltage for a period of time.
Boost converters must also deal successfully with several other detrimental operational characteristics. One of these is "inrush" current, which is the potentially damaging initial, transient current that may flow into the boost converter when the AC power is first turned on. At AC power turn-on, an inrush current that is not limited in some way is typically much larger than the currents which flow during normal, steady-state operation. This unlimited inrush current may cause circuit element damage unless more robust components are selected to accommodate these larger values of transient current which normally leads to sacrifices in either component cost or size. Alternately, the nature of the boost converter design itself may allow for limiting or accommodation of the inrush current.
Another potentially detrimental operating characteristic occurs when the AC voltage input to the boost converter varies in a transient manner causing momentary fluctuations from its normal sinusoidal waveform. These fluctuations may instantaneously "surge" to higher values than expected as various other loads are added or removed from the AC line. These voltage perturbations, if not adequately isolated, may negatively affect boost converter operation.
Accordingly, what is needed in the art is a way to efficiently accomplish holdup mode operation in a boost converter that also effectively limits the effects of AC supply line inrush currents and accommodates AC supply line surge voltages.